In the process of producing electricity, power generators also create heat that must be dissipated away from the generator. Heat occurs in generators due primarily to friction and current. Frictional heating occurs as the rotor spins at high speed in the generator. Similarly, heating also occurs as current flows through the rotor and stator coils, as these coils rotate relative to one another in the magnetic fields of the generator. Generators are equipped with cooling systems to transfer heat from the stator and rotor out away from the generator.
Generators are traditionally gas-cooled by ventilated cooling systems that circulate air, or other cooling gases, through ducts in the rotor and stator. In this regard, FIG. 1 shows a cross-section of one-half of a generator 4 (see axial center-line 6 and longitudinal center-line 8) having a conventional reverse flow ventilated cooling system 10. A portion of the flow of cooling gases is directed to the rotor 12 where cool gases 14, such as ambient air, are drawn into the rotor body. The cool gases 14 are drawn through ventilation ducts 16, 18 in the rotor by centrifugal forces created by the spinning rotor. As the gases flow through the rotor, heat in the rotor is transferred to the gases as the temperature of the gases rises towards the temperature of the rotor coils 20. The heated rotor gases exit the ducts at the surface of the rotor into an air gap 22 between the rotor and stator 26. Moreover, the rotor may be cooled in many ways, other than the exemplary manner shown in FIG. 1. Spinning fans 28 mounted at the ends of the rotor draw the heated gas through the gap 22 between the stator and rotor. The heated gas 24 is directed by an external duct 29 to a heat exchanger 28 that cools the gas.
The stator 26 is cooled by ventilation flow paths that are separate from the flow paths in the rotor. Cold gas 30 cooled by the heat exchanger 28 enters a plenum chamber 32 surrounding the stator. Because the end sections of the stator are closest to the rotor exhaust fans 28, cooling gas tends naturally to flow in greater volume and velocity through the ducts near the ends of the stator than through ducts at the center of the stator. This potential unbalance in the flow of cooling gas through the stator has traditionally been compensated for by baffle chambers 34 that extend around outer surface of the stator. The baffle chambers are arranged to distribute cooling gas uniformly along the length of the stator such that the flow of gas to the center section 36 of the stator is substantially equal to the flow of gas to stator ends 38. The baffle chambers impose additional flow restrictions and pressure drops on the cooling gas entering the end sections of the stator which cause some cooling gas that would naturally flow to the ends of the stator to flow to the center of the stator. However, the pressure drops in the cooling gas caused by the baffle chambers reduce the gas pressure available to move the cooling gas through the ventilation ducts of the stator.
The cooled gas 30 pass through the baffle chambers (or directly to the stator near the center of the stator) and into cooling gas intake ducts 40 in the outer circumferential surface 42 of the stator 26. In conventional reverse flow systems, the stator ducts 40 are uniform in cross-sectional area, length and spacing (frequency). As the gas flows radially inward through the stator, heat from the stator coils 44 is transferred to the gas. The rotor fans 28 draw the warmed gas from the stator, into the air gap 22 and out to the external duct 29 to the heat exchanger 28. A portion of the cooled gas 30 from the heat exchanger is exhausted from exhaust ports 46 in the ends of the plenum chamber 32 around the stator to cool the stator end turns 48.
Relatively high ventilation pressure heads are needed to pump cooling air through the baffle chambers 34 and stator ducts 40 of current large power generators. As the power generator ratings and sizes increase, the requirement for higher ventilation pressure heads for cooling gas increases as well. Some of the pressure head of the cooling gas is dissipated by the baffle chambers and, thus, the pressure head must be increased to compensate for the loss in the baffles. One solution to this difficulty is to use other types of cooling gases, e.g., hydrogen gas, that have a higher cooling capacity than ambient air. However, these other types of gases require special seals, purging systems and diagnostic instruments that are complex and expensive.
Air is a simple to use and cheap cooling gas, that does not require complex systems to handle. Unfortunately, air has a relatively low cooling capacity. To compensate for the low cooling capacity of air, the volume of air flow through the stator has been progressively increased with better rotor fans that provided high pressure heads to the cooling gases. Pressure heads could be further increased with multi-stage fans, but power consumption increases. As a result, overall efficiency decreases, and costs and complexity would increase. In addition, the baffles used to direct the flow of cooling gases toward the center of the stator deflect the flow of the gases and, thus, have the undesirable effect of reducing the pressure of the gases. Accordingly, there is a long-felt need to increase the efficiency of air cooled, reverse flow generators.
The current invention is a reverse flow, ventilated cooling system for a generator in which the cooling ducts in the stator are arranged to provide optimal flow of cooling gases through the stator. For example, the spacing and cross-sectional area of stator cooling ducts are varied along the length of the stator to optimize the distribution of cooling gases through the stator and minimize the necessary pressure head needed for the cooling gases. It is an object of the invention to provide a reverse cooling system having reduced pressure head requirements at the stator duct intakes. Another object of this invention is to provide cooling ducts in stators having reduced pressure drops through the stator ducts. A further object of the invention is to provide stator cooling ducts arranged such that the maximum pressure drop of the cooling gases, and hence maximum gas velocity, occurs in the areas of the stator that most require cooling. Moreover, another object of the invention is to provide stator cooling ducts that intensify the cooling action in the sections of the stator that most require cooling.